1. Field of the Invention
The present invention relates to a combustion control device and a combustion control method for a spark ignition direct injection engine. More particularly, the present invention pertains to a combustion control device and a combustion control method that promote the vaporization and radicalization of fuel injected from a fuel injection valve into a cylinder taking advantage of the heat of exhaust gases (burned gases), thereby improving the combustibility, fuel economy, exhaust gas purification performance, and the like.
2. Description of the Related Art
The spark ignition direct injection engine injects fuel directly into each cylinder. Accordingly, the spark ignition direct injection engine injects the fuel in the latter half of a compression stroke, and during a stratified charge combustion, in which an air-fuel mixture is burned in the cylinder with the air-fuel ratio distribution of the mixture therein made uneven, it can vaporize the fuel by compressed air in the cylinder. Simultaneously, the high pressure within the cylinder decreases the penetration force of a mist, thereby reducing the adhesion of the fuel to the wall surface of the cylinder.
In the homogeneous combustion (stoichiometric combustion), in which fuel is burned in the vicinity of the theoretical air-fuel ratio, the fuel is injected in the first half of an intake stroke in order to buy the mixture time for making uniform the air-fuel mixture in the cylinder. However, during the intake stroke, since the pressure within the cylinder becomes equal to the pressure within an intake pipe, the cylinder temperature falls especially during cold start, so that the amount of heat required to vaporize the fuel cannot be obtained. As a result, the air-fuel mixture is prone to become non-uniform due to a lack of vaporization, thereby causing the deterioration of exhaust gas purification performance and fuel economy.
Such being the situation, when attempting to produce a homogeneous air-fuel mixture, it is necessary to raise the temperature within the cylinder and secure the amount of heat required for fuel evaporation (vaporization). As one solution to this issue, JP-A-2002-256924 discloses a method by which the exhaust valve is early closed in course of the exhaust stroke; fuel is injected into burned gases having residual oxygen; the fuel is ignited in the vicinity of the end of the exhaust stroke; and the intake air is heated by the heat due to the ignition, thereby promoting the vaporization of the fuel, and enhancing the pressure within the cylinder during the compression stroke (see pages 1 to 5, and FIGS. 1 to 6 of the above patent document).